It is proposed to continue the studies of this laboratory on several cellular control mechanisms. Particular attention will be paid to the events taking place in embryonic systems, such as the developing chicken embryo. In this system the development of the various DNA and RNA polymerases will be followed at several developmental stages, with particular emphasis on differences in the template requirements of these enzymes at various periods of growth and on divergences between the products made by the enzymes. Parallel studies with Rous sarcoma material are also contemplated. Other investigations will deal with the biological role of ribonuclease H, the possible significance of apurinic acid sites in DNA replication, and with the control functions of nucleotide phosphotransferases, of a newly discovered purine polyribonucleotide synthetase, and of poly(adenosine diphosphate ribose).